Why TSH is the monitored line
TSH is the most sensitive barometer of therapy: the pituitary reacts to the slightest shortage or excess of thyroxine long before symptoms appear. When the dose is right, TSH returns to its target range; when it is not, the doctor adjusts the dose by that line.
The key rule is patience: TSH resets slowly, so a test earlier than 6–8 weeks after a dose change shows the old picture and provokes an unnecessary correction. Wait for the date — earlier is not better here.
How to test while on the medication
Blood for TSH is drawn in the morning before the tablet — the result then reflects the baseline without the wave from the morning dose. The tablet itself is taken fasting with water, 30–60 minutes before breakfast: coffee, calcium, iron, and omeprazole block absorption and eat part of the dose.
If you start calcium or iron supplements, separate them from thyroxine by at least 4 hours and tell the endocrinologist: the dose may need recalculating.
When T4, T3, and antibodies are needed
Free T4 joins TSH in special situations: pregnancy (TSH targets there are different and stricter), pituitary disease (when TSH lies), and dose-finding with an unusual picture. Free T3 is almost never needed in routine levothyroxine monitoring.
TPO antibodies are worth testing once — they confirm autoimmune thyroiditis as the cause of hypothyroidism. Tracking their level over time is pointless: the titer does not change treatment.
Long-term companions: bones and heart
Years of excessive dosing speed up bone turnover and load the heart — one more reason to keep TSH on target rather than "a bit low for energy." With long-term therapy, especially in women after menopause, the doctor monitors calcium and vitamin D.
And remember weight: the thyroxine dose is tied to body mass. Losing or gaining 10+ kilograms, pregnancy, or starting hormones are reasons for an unscheduled TSH — not for waiting for the annual checkup.
A monitoring schedule on levothyroxine
- TSH — 6–8 weeks after starting or changing the dose
- TSH — every 6–12 months on a stable dose
- Free T4 — in pregnancy and pituitary disease
- TPO antibodies — once, to confirm the cause
- Calcium and vitamin D — with years of therapy, as ordered
Markers covered in this article
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
Frequently asked questions
TSH is normal but I feel bad — raise the dose?
Can I stop levothyroxine once results are good?
Forgot a tablet — what now?
Sources
- TSH (Thyroid Stimulating Hormone) Test: MedlinePlus Medical Test — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.
- TSH test: MedlinePlus Medical Encyclopedia — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.
- Thyroid Antibodies: MedlinePlus Medical Test — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.
- Autoimmune Thyroiditis: ARUP Consult — ARUP Consult, ARUP Laboratories. Accessed: August 29, 2026.